1. Field of the Invention
Exemplary embodiments of the present invention relate to a method for transmitting data frames to multi-user in a wireless communication system; and, more particularly, to a method capable of reconfiguring a group according to the channel states of plural stations (STAs) and the size of data to be transmitted and deciding transport stream numbers of the STAs composing the group such that the transport stream numbers of the respective STAs, which are contained in a signal field, may be expressed as a minimum bit number, in a multi-user based wireless communication system.
2. Description of Related Art
In general, a wireless communication system such as wireless LAN includes a plurality of access points (AP) connected to each other through a network. A plurality of STAs included in a basic service set (BSS) managed by one AP simultaneously transmit and receive frames to and from the AP. Meanwhile, an independent BSS (IBSS) includes a plurality of STAs, and one STA simultaneously transmits and receives frames to and from the other STAs.
Recently, much attention has been paid to not only MIMO (Multiple Input Multiple Output) technology for transmitting multiple streams only to one STA, but also MU-MINO (Multi-User MIMO) technology for transmitting multiple streams to a plurality of users.
The MU-MIMO technology refers to a method in which an AP removes the interference between a plurality of STAs to transmit data, and may acquire even a multi-user diversity gain with an increase in channel capacity of multiple antennas. The MU-MIMO technology may simultaneously transmit multiple streams to multiple users by using the same frequency band. Therefore, the MU-MIMO technology may efficiently improve throughput while increasing the complexity of the AP.
In order to increase the efficiency of an information transmission method in a wireless communication system to which the MU-MIMO is applied, a plurality of STAs within an AP may be mapped into a plurality of groups by using information such as channel characteristics. The group information in such an AP may be transmitted to the STAs such that each of the STAs may recognize a group to which the STA belongs and its order within the group.
FIG. 1 is a diagram explaining a conventional process for transmitting a data unit in the above-described wireless communication system.
Referring to FIG. 1, the AP transmits information through a control information frame 101. The information may include information which is required when the plurality of STAs transmit channel sounding/channel feedback Sounding/Feedback 102 and information which is required for receiving data. In other words, the AP requests channel sounding or channel feedback for the plurality of STAs by using the control information frame 101.
Each of the STAs receiving the control information frame 101 from the AP transmits channel information to the AP in response to the channel sounding or channel feedback request (102). At this time, the STA may divide the channel information in a temporal or spatial manner to transmit the channel information or divide the channel information for each subcarrier to transmit the channel information.
The AP receiving the channel information from the respective STAs decides the number of STAs capable of simultaneously transmitting data and transport stream numbers for the respective STAs, by considering the correlation between channels and/or the QoS (Quality of Service) of data. Furthermore, the AP transmits data frames for the respective STAs with the information on the STAs which are to simultaneously transmit data (103). Here, the AP transmits the information on the STAs which simultaneously transmit data and the information on transport stream numbers for the respective STAs to the STAs through a common signal which may be received by all the STAs.
Each of the STAs receiving the data frames demodulates the received data frame by using the information on the STAs which simultaneously transmit data and the information on the transport stream numbers for the respective STAs, which are contained in the common signal transmitted from the AP. Then, the STA checks whether or not an error occurred in the received data, and transmits an ACK frame indicating a check result to the AP.
When an AP simultaneously transmits frames to a plurality of STAs, it is referred to as multi-user transmission. In order to perform the multi-user transmission, the AP allocates the plurality of STAs existing within its region as a plurality of groups, based on channel characteristics and the QoS. The information on the allocated groups is transmitted to the respective STAs through a control information frame of a MAC layer.
FIG. 2 shows an example of a group definition field contained in the control information frame for informing the respective STAs of the group information.
A group identifier (ID) Group ID having arbitrary y bits is used to specify K STAs (K is a natural number, for example, K≦4) according to a specific sequence. The group ID is defined by the AP and transmitted to the STAs. A plurality of group IDs Group ID may be allocated to one STA. That is, one STA may belong to a single group or a plurality of groups.
The group definition field includes information on STAs belonging to one group ID Group ID and the sequence of the STAs. Referring to FIG. 2, the group definition field includes a group ID having arbitrary y bits and ID information STA 1 ID to STA K ID of STAs belonging to the same group. Each of the STAs may recognize the group ID of the group to which the STA belongs and the order of the STA within the group, when receiving the group definition field. Since the transport stream numbers of the respective STAs for each group are required for demodulating a long training field (LTF) for channel estimation, the information on the transport stream numbers of the STAs should be contained in control information for data transmission.
In a recent VHT (Very High Throughput) wireless LAN system, the MU-MIMO technology is applied to consider a protocol data unit (PPDU) format as shown in FIG. 3.
FIG. 3 shows a PPDU format in an environment in which legacy STAs, HT (High Throughput) STAs, and VHT STAs are mixed. Referring to FIG. 3, the VHT PPDU includes training symbol fields L-STF and L-LTF for legacy STAs, a legacy-STA signal field L-SIG, a signal field VHT-SIG-C for VHT STAs which may be decoded by all STAs, training symbol fields VHT-STF and VHT-LTF for VHT STAs, a VHT-STA signal field VHT-SIG-D which may be decoded only by VHT STAs, and a data field Data.
In FIG. 3, fields from the training symbol field L-STF to the signal field VHT-SIG-C may be received by the VHT STAs as well as the legacy STAs and the HT STAs, and the following fields from the training symbol field VHT-STF may be received only by the VHT STAs. Therefore, the transport stream numbers (LTF symbol numbers) of all the VHT STAs within the group should be contained in the signal field VHT-SIG-C by using predetermined bits, a predetermined modulation scheme, and a predetermined coding scheme.
When the number of STAs composing a group which is divided for multi-transmission is set to K, a value obtained by summing the numbers of transport streams which may be transmitted by K STAs is smaller than the number of streams which may be simultaneously transmitted by the AP. For example, when it is assumed that the number of STAs composing one group is eight and each of the STAs may transmit a maximum of four streams, the maximum number of transport streams which may be transmitted by eight STAs composing the group becomes 32.
When it is assumed the STAs know only the group information from the control information frame transmitted from the AP, all possible combinations of the numbers of transport streams which may be transmitted by the eight STAs composing the group should be transmitted to the STAs through a common signal. All the possible combinations of the numbers of transport streams which may be transmitted by the eight STAs composing the group may include a plurality of cases from a case in which no stream is transmitted to a case in which the maximum of four streams are transmitted.
That is, each of the STAs may receive any one transport stream from 0 to 4. Considering such a case, three bits are required to express the transport stream number for each STA. When it is assumed that the transport stream information is transmitted to each STA, the information to be contained in the signal field VHT-SIG-C should have a bit number shown in FIG. 4. FIG. 4 shows the bit number for expressing the transport stream number of each STA, when the number of STAs composing one group is eight. In this case, a total of 24 bits are required to express the transport stream numbers of all the STAs within the group.
In the multi-user transmission, multi-user diversity will increase as the number of STAs within the group increases. The increase in the number of STAs composing the group may cause an increase in the number of bits representing the transport stream numbers of the respective STAs, which are to be contained in the signal field VHT-SIG-C. In general, the total bit number of the signal field VHT-SIG-C is fixed, and the signal field VHT-SIG-C includes a variety of different information as well as group information. Therefore, a transport stream number field defining the transport stream numbers of the STAs corresponding to a group ID needs to be simply expressed by using a small bit number.